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Abnormal sweat test

My 6 year old son has always been ill. He has had three sinus surgeries, and is in the 5 percentile for weight and height. He has had RSV and bronchitis more than once and been to the er with stomach pains several times. He has asthma and chronic sinusitis . He had a sweat test with the result of 53. We have an appt for a second sweat test. What is the likelyhood that he will be diagnosised with a form of cf?
Dear Parent

Thank you for your question regarding your child.
The diagnosis of CF is usually made on the basis of several tests. Sometimes the diagnosis is clear cut when a person has two CF causing mutations and a clearly positive sweat test. However as there is a complete spectrum of disease people at the milder end of the spectrum may be more difficult to diagnose conclusively.
In these cases it is a combination of the sweat test, genetics, faecal elastase level,NPD(nasal potential differences), family history and symptoms, which may help to decide the diagnosis.
Certain mutations in CF are known to be associated with borderline or normal sweat tests eg R117H, D115214, G551S. At times those at the milder end of the spectrum with a borderline sweat test result may become more positive over time. Those with a borderline sweat test do need further investigation and ongoing follow up.

If your doctor is planning to carry out further testing this is likely to involve repeating the sweat test, sending blood for DNA analysis for the most common CF mutations, sometimes nasal potential differences are checked and faecal elastase samples can also be sent.
If the sweat test remains borderline and the genetics do not reveal two mutations the diagnosis may remain inconclusive.
Your doctor may decide that the diagnosis is in keeping with CFTR related disorder / CF related metabolic disorder.
Certainly your child does seem to have had a lot of symptoms and with an inconclusive sweat test it is appropriate for further investigations to be carried out. Please discuss this with your local clinician.
I hope you get definitive diagnosis for your child.

Dr Laura Jenkins
CF Associate Specialist Doctor(Paediatrics)

1.Guidelines for the diagnosis of Cystic Fibrosis in newborns through to older adults : CFF consensus report
J. Pediatr 2008, August 153(2) S4-S14 (can be accessed on line)

The answer is edited by: Laura Jenkins